1. Field of the Invention
This invention relates to a process for the improvement of the flow characteristics of slurries of solid fuels in water. More particularly it is concerned with a process for the production of coal water slurries which are made better Bingham plastics by the incorporation therein of a prescribed group of additives as hereinafter more fully described. Thereby reduced viscosities are obtained at high rates of applied shear. This fact in turn is advantageous both by virtue of the savings in pumping energy which are effected as well as by virtue of the longevity which is imparted to the pumping equipment involved.
Most solid carbonaceous fuels, as mined, generally contain varying amounts of water, which in some instances may be as high as 40 wt. % or even higher in the case of low grade solid fuels. This water is an undesirable constituent of the fuel, particularly in the case of fuels of high water content. Thus a slurry containing 50 wt. % water and 50 wt. % solid fuel would contain considerably less than that amount of fuel when the fuel is measured on a dry basis.
The amount of water necessary to form a pumpable slurry depends on the surface characteristics of the solid fuel. For example, soot formed during the partial oxidation of a carbonaceous material has such a high surface area that a concentration of such soot in water in excess of a few wt. % renders the resulting slurry umpumpable. In the case of a slurry which is to be fed to a gas generator, it is necessary that the solid fuel be ground to such an extent that a major portion thereof will pass through a 60 mesh sieve so that the particles are substantially completely converted to oxides of carbon during their short residence time within the gasification zone. However, ordinarily before reaching the gasification zone the slurry must pass through various pieces of equipment such as heat exchangers and compressors on its way from the slurry zone to the gas generation zone. Accordingly the slurry must be pumpable but in the case of a slurry made up of solid fuel particles most of which will pass through a 200 mesh sieve it has been found that ordinarily, a pumpable slurry must contain from about 55 to 60 wt. % water. Unfortunately a slurry containing this amount of water renders the operation of the gasifier unsatisfactory as this excessive amount of water moderates the temperature of the reaction zone to such an extent that it seriously affects its thermal efficiency. It has been found that the optimum amount of water in a solid fuel-water slurry which may be used as feed to a gas generation zone will lie from between 40 and 50 wt. %. A water content of 30 wt. % would be even more preferable, if it could be achieved.
It is therefore an object of this invention to produce solid fuel water slurries having a relatively high solids content. Still another object of the invention is to produce pumpable slurries of solid fuel in water wherein the bulk of the solid fuel will pass through a sieve as fine as 60 mesh and in which the water content of the slurry will range between about 40 and 50 wt. %. Yet another object of the invention is to produce an improved Bingham plastic slurry, which by definition is handled with lower expenditure of energy as a result of lowered viscosity at high shear rates which may be expected at high rates of flow. Yet another object of the invention is to extend the useful life of pumping equipment by lowering the viscosity of the slurries pumped when operating at customary shear rates. These and other objects will become apparent on further reading of this specification.
2. Description of the Prior Art
U.S. Pat. No. 3,210,168 discloses the use of an imidazoline surfactant to stabilize a dispersion of pulverized coal coated with a liquid hydrocarbon fuel in about 15 to 20% by weight of water.
U.S. Pat. No. 3,907,134 discloses the use of detergents, lignin liquors or amino acids as viscosity reducing agents for incorporation in fuel-water slurries comprising a solid particulate carbonaceous material and a liquid hydrocarbon fuel that also contains a slurry suspension stabilization agent such as starch.
U.S. Pat. No. 4,088,453 discloses the use of between about 0.1 and 5.0 wt. % of ammonia to improve the pumpability of a solid fuel-water slurry by reducing the slurry viscosity.
Japanese patent application Derwent Accession No. 61415A discloses the use of one or more bisamides of the general formula EQU R.sub.1 CONH--R.sub.3 --N(R.sub.5)--R.sub.4 --NHCOR.sub.2
wherein both R.sub.1 and R.sub.2 are C.sub.11 alkyl or alkenyl groups, both R.sub.3 and R.sub.4 are C.sub.2 -C.sub.4 alkyl groups and R.sub.5 is H or a C.sub.1 -C.sub.4 alkyl group, as an emulsifier for coal oil based mixed fuel slurries.
Japanese patent application, Derwent Accession No. 61418A discloses the use of one or more ether amine derivatives of the general formula ##STR1## wherein R is a greater than C.sub.8 alkyl or alkenyl group, m and n are integers in the range between 0 to 20 and the sum of m+n is less than 30.
A further Japanese patent application, Derwent Accession No. 55787A discloses the use of a compound related to the one mentioned in the preceding paragraph or a salt thereof wherein the alkyl ether group attached to the amino group is a C.sub.12+ alkyl or alkenyl group, m and n are integers in the range from 0 to 20 and the sum thereof is 1-30.